(1) Field of the Invention
The present invention relates to a pneumatic tire having an improved tread, and more particularly to a pneumatic tire for travelling on frozen roads, which has a tread consisting of an improved rubber composition having a high friction coefficient against frozen road surface.
(2) Description of the Prior Art
There has hitherto been proposed a so-called spiked tire having a tread provided with spikes, or a tire united with a chain in order to travel safely on a frozen road. However, when these tires with spikes or a chain travel on a paved road not covered with snow or ice, the road surface is damaged or heavy noise is produced to cause serious public problems due to the contact of the metals, such as spikes and chain, with the road surface. In order to solve these problems, the development of pneumatic tires for travelling on frozen roads, which do not require spikes or a chain, has been demanded.
In order to satisfy the demand, the inventors have made various investigations and found out that, when the frictional force between the tread rubber of a tire and the road surface is high, the tire can travel safely on frozen roads without the use of spikes or a chain, and that the use of a rubber composition having an improved property in the tread is effective for this purpose.
It is generally considered that friction between the tread rubber of a tire and the common paved road surface not covered with ice and snow is caused by the energy loss of the rubber and the sticking force of the rubber with the road surface, and that a rubber having the higher energy loss and sticking force has the higher friction coefficient. However, it is also considered that, when a paved road surface is wet, sticking force does not substantially act between a rubber and the road surface due to the presence of water film, and the friction coefficient of the rubber against the wet paved road surface depends mainly upon the energy loss of the rubber. Therefore, it is necessary that the energy loss of rubber must be large in order to increase the friction coefficient of the rubber. However, deformation of rubber generally becomes difficult at low temperature produced, for example, on the frozen road due to remarkably poor movability of rubber molecule at such low temperature. Therefore, the contribution of the energy loss of rubber to the increase of the friction coefficient of the rubber is very small. Accordingly, when a rubber easily deformable even at low temperature is used to increase its contact area with ground and to improve its sticking force to ice in the presence of water film, the friction coefficient of the rubber against frozen road surface can be increased. The inventors have found out that, when a low temperature-resistant rubber composition containing at least polyisoprene rubber as a rubber component and having a low JIS hardness at low temperature, a low tan .delta. peak temperature and an improved affinity for water is used in the tread of a tire, the friction coefficient of the tire against frozen road surface is high and the tire can travel safely on frozen roads, and accomplished the present invention.